1. Field of the Invention
This invention relates to thermal reference control for night vision systems.
2. Brief Description of the Prior Art
Due to the non-linearity of infrared detectors as used in infrared night vision systems as a function of temperature, such systems generally utilize one or two thermal reference sources to compensate for such non-linearity in known manner. Such compensation is commonly known as normalization. One of the thermal sources, herein referred to as the level thermal reference, is generally adjusted to provide a temperature equal to the average temperature of the scene being scanned by the detector. The other thermal source, herein referred to as the gain thermal reference, is adjusted to a temperature which is a known amount different from the average temperature of the scene, e.g. +10.degree. C. greater.
Prior art infrared night vision systems using active normalization techniques have not addressed problems of inherent non-linearities of the detector, thermal reference sources, optics, and video chain electronics as limitations in the normalization process. Since each of these elements resides within the path of the thermal reference control loop (see, for example, FIG. 1), they negatively impact the transient response (or slew rate) of the thermal reference source and thus have negative implications on the overall night vision system performance. Because the detector, in particular, inherently changes gain as a function of the scene temperature, failure to compensate for this detector non-linearity results in sluggish settling time of the thermal reference control loop. The failure is due to control loops that do not account for the aforementioned non-linearities, especially in the detector. The sluggish settling time results in inferior night vision system performance due to image degradation. Image degradation can occur in the form of image "wash out" or poor image contrast following large scene temperature changes. Large scene temperature changes can be induced through physical changes in the field of view or from changes in scene temperature within a static field of view.
Image wash out can result in unintelligible image conditions. For example, the operator of a night vision system may slew from the sky with an apparent average scene temperature of -30.degree. C. to a ground surface with an apparent average scene temperature of 50.degree. C. Upon performing a physical transition from viewing the sky to viewing the ground, intelligibility of the image will disappear for a period of time. This poses potentially adverse consequences to the operator. During such dynamic scene changes, the detector gain continually changes as a non-linear function of scene temperature. Left uncorrected, the gain variation adversely affects the slew rate performance of the thermal reference, resulting in less than optimal duration of image degradation. Since the thermal reference source is attempting to track and equal the required average scene temperature, minimizing the recovery time from an unintelligible image back to an intelligible image is largely dependent upon continually optimizing the bandwidth of the thermal reference control loop. Applicants are unaware that this problem has been recognized or solved in the prior art.